With the decrease of the oil and coal stocks and increase of the human energy consumption, energy saving has become a consensus in the 21st century. According to the estimation by the U.S. department of energy (DOE), about two-thirds of the electricity is used for motor drivers. Since semiconductor power devices such as IGBTs (insulated gate bipolar transistors) and the matching RFDs (fast recovery diodes), which are mainly used as high frequency rectifier diodes, free-wheeling diodes or damping diodes in switching power supplies, PWM width modulators, frequency converters and other electronic circuits, can save the energy consumption of the motor driver by 20%-30%, it can be expected that the application of the power devices will get a rapid growth in the future.
PN junctions are elementary “building blocks” for the structure of a power device. The PN junction refers to a space charge region formed at the interface between the P-type and N-type semiconductor materials inside a same semiconductor substrate (usually a silicon substrate or a germanium substrate) by doping and diffusion of dopants.
However, since the junction depth of the diffused PN junctions is usually a few microns, the small radius of the junction curvature may cause electric field intensity, thereby making the breakdown voltage far below the planar junction breakdown voltage. For example, for a device having a P-N junction of 5 microns deep, the planar junction breakdown voltage thereof is assumed to exceed 1200V however, the actual breakdown voltage may be lower than 400V due to the electric field intensity caused by the junction curvature, which is far below the planar junction breakdown voltage. Therefore, guarding rings are needed in the periphery regions of the device. The structure of a field plate combined with field limiting rings is an earlier developed guarding ring and is widely applied so far due to its simple manufacturing process without additional photoetching steps.
FIG. 1 illustrates a sectional diagram of a conventional field plate and field limiting ring structure. The middle region in FIG. 1 refers to the device region 9, the guarding ring is composed of diffusion rings 5 which are P+ type diffusions formed in an N type monocrystalline silicon body 3, and an outmost equipotential ring 4 which is an N+ type diffusion region and is coupled to a high voltage or floated when the device region 9 works. The guarding ring is a symmetric structure around the device region 9 according to its cross-section. The regions between the two P+ type diffusion rings outside the device region 9 refers to the main withstand voltage region. The P type diffusion ring 5 adjacent to the device region 9 refers to an innermost ring, which is grounded when the device region 9 works. The P+ type diffusion ring 5 surrounding the innermost ring refers to a first ring which is floated when the device region 9 works. The number of the P+ type diffusion rings 5 (the field limiting rings) as shown in FIG. 1 can be added and thus a second ring, a third ring, a fourth ring and even more rings can be formed to satisfy the withstand voltage requirements. The manufacturing process of the guarding ring comprises: performing photoetching to form the pattern of the P+ type diffusion ring regions and implanting to form the diffusion rings 5; then performing photoetching to form the pattern of an equipotential ring region and implanting to form the equipotential ring 4; and performing well drive-in.
FIG. 2 illustrates a detailed sectional diagram of the conventional field limiting ring structure. Wherein, the fine dash line refers to the boundary of the depletion region, the dot and dash line refers to the electric field line. The conventional field limiting ring structure illustrated in FIG. 2 forms a triangle electric field distribution.
FIG. 5 is a comparison diagram illustrating the electric field distribution formed underlying the field oxide layer in the conventional field limiting ring and that foamed in the structure of the present invention. The triangle profile represents the electric field distribution in one ring spacing of the field limiting ring structure. As shown in FIG. 5, the electric field distribution of the conventional field limiting ring structure forms a triangle profile, wherein the left side of the triangle with higher slope represents the electric field at the P type depletion region inside the P+ type diffusion ring 5 region and the right side of the triangle with lower slope represents the electric field at the N type depletion region between the two diffusion ring 5 regions.
The magnitude of the electric field slope relates to the doping concentration. The higher the doping concentration is, the greater the slope will be. Since the P+ type doping concentration in the diffusion ring 5 is always higher than the N type doping concentration in the region between the diffusion rings 5, the slope of the left side of the triangle is higher than that of the right side, the peak of the triangle represents the breakdown electric field and the area of the triangle represents the withstand voltage.
Those skilled in the art know that with the increase of the withstand voltage, more P+ type diffusion rings 5 are needed (For example, a field limiting ring structure generally utilizes 4 rings to achieve 1200V withstand voltage, while utilizes 22 rings to achieve 3300V withstand voltage). Therefore, it requires more guarding ring area and costs more time to design the guarding ring.
Accordingly, in the conventional fielding limiting ring structure, the ring space is not fully utilized as the electric field distributes in a triangular profile. How to make full use of the ring spacing so as to achieve the same withstand voltage with reduced ring spacing, or to enable each ring spacing to achieve a higher withstand voltage and thus reduce the ring number is an urgent problem to be solved in the industry.